Proper functioning of the vascular system is essential for the health and fitness of living organisms. The vascular system carries essential nutrients and blood gases to all living tissues and removes waste products for excretion. The vasculature is divided into different regions depending on the organ systems served. If vessels feeding a specific organ or group of organs are compromised, the organs and tissues supplied by those vessels are deleteriously affected and can even fail completely.
Vessels, especially various types of arteries, not only transmit fluid to various locations, but are also active in responding to pressure changes during the cardiac cycle. With each contraction of the left ventricle of the heart during systole, blood is pumped through the aorta and then distributed throughout the body. Many arteries contain elastic membranes in their walls that assist in expansion of the vessel during systole. These elastic membranes also function in smoothing pulsatile blood flow throughout the vascular system. The vessel walls of such arteries often rebound following passage of the systolic pressure waveform.
In autoregulation, cerebral blood vessels maintain constant cerebral blood flow by either constricting or dilating over a certain mean arterial blood pressure range so that constant oxygen delivery is maintained to the brain. Vascular failure occurs when the pressure drops too low and the oxygen delivery starts to fall. If the blood pressure gets too high and the vessels can no longer constrict to limit flow, then hyperemia breakthrough or loss of autoregulation can occur. Both of these conditions are pathologic states, and have been described in the literature in terms of mean arterial pressure and cerebral blood flow velocity, but there are others that cannot be explained based on that model. The failure of the model is that it relies upon systemic blood pressure. The pressure of blood in the brain itself is not being measured directly. The resultant pressure curve has an S-shaped curve.
The force applied to the blood from each heartbeat is what drives the blood forward. In physics, force is equivalent to mass times acceleration. But when blood is examined on a beat-to-beat variation, each heartbeat delivers about the same mass of blood, unless there is severe loss of blood or a very irregular heart rhythm. Therefore, as a first approximation, the force of flow on the blood at that particular moment is directly proportional to its acceleration.
Diseased blood vessels lose the ability to stretch. The elasticity or stretch of the blood vessel is very critical to maintaining pulsatile flow. When a muscle is stretched, it is not a passive relaxation. There is a chemical reaction that happens within the muscle itself that causes a micro-contracture to increase the constriction, so that when a bolus of blood comes through with each heartbeat, it stretches the blood vessel wall, but the blood vessel then contracts back and gives the kick forward to maintain flow over such a large surface area. This generates a ripple of waves, starting in the large vessel of the aorta and working its way through the rest of the vessels. As vessels become diseased, they lose the ability to maintain this type of pulsatile flow.
Further, if vessels are compromised due to various factors such as narrowing or stenosis of the vessel lumen, blood flow becomes abnormal. If narrowing of a vessel is extensive, turbulent flow can occur at the stenosis resulting in damage to the vessel. In addition, blood cannot flow adequately past the point of stenosis, thereby injuring tissues distal to the stenosis. While such vascular injuries can occur anywhere throughout the body, the coronary and cerebral vascular beds are of supreme importance for survival and well-being of the organism. For example, narrowing of the coronary vessels supplying the heart can decrease cardiovascular function and decrease blood flow to the myocardium, leading to a heart attack. Such episodes can result in significant reduction in cardiac function and death.
Abnormalities in the cerebral vessels can prevent adequate blood flow to neural tissue, resulting in transient ischemic attacks (TIAs), migraines, and stroke. The blood vessels that supply the brain are derived from the internal carotid arteries and the vertebral arteries. These vessels and their branches anastomose through the great arterial circle, also known as the Circle of Willis. From this Circle arise the anterior, middle and posterior cerebral arteries. Other arteries such as the anterior communicating artery and the posterior communicating artery provide routes of collateral flow through the great arterial circle. The vertebral arteries join to form the basilar artery, which itself supplies arterial branches to the cerebellum, brain stem and other brain regions. A blockage of blood flow within the anterior cerebral artery, the posterior cerebral artery, the middle cerebral artery, or any of the other arteries distal to the great arterior circle results in compromised blood flow to the neural tissue supplied by that artery. Since neural tissue cannot survive without normal, constant levels of glucose and oxygen within the blood and provided to neurons by glial cells, blockage of blood flow in any of these vessels leads to death of the nervous tissue supplied by that vessel.
Strokes result from blockage of blood flow in cerebral vessels due to constriction of the vessel resulting from an embolus or stenosis. Strokes can also arise from tearing of the vessel wall due to any number of circumstances. Accordingly, a blockage can result in ischemic stroke depriving neural tissue distal to the blockage of oxygen and glucose. A tearing or rupture of the vessel can result in bleeding into the brain, also known as a hemorrhagic stroke. Intracranial bleeding exerts deleterious effects on surrounding tissue due to increased intracranial pressure and direct exposure of neurons to blood. Regardless of the cause, stroke is a major cause of illness and death. Stroke is the leading cause of death in women and kills more women than breast cancer.
Currently, more than three-quarters of a million people in the United States experience a stroke each year, and more than twenty-five percent of these individuals die. Approximately one-third of individuals suffering their first stroke die within the following year. Furthermore, about one-third of all survivors of a first stroke experience additional strokes within the next three years.
In addition to its terminal aspect, stroke is a leading cause of disability in the adult population. Such disability can lead to permanent impairment and decreased function in any part of the body. Paralysis of various muscle groups innervated by neurons affected by the stroke can lead to confinement to a wheelchair, and muscular plasticity and rigidity. Strokes can leave many patients with little or no ability to communicate either orally or by written means. Often, stroke patients are unable to think clearly and have difficulties naming objects, interacting well with other individuals, and generally functioning within society.
Despite the tremendous risk of stroke, there are presently no convenient and accurate methods to access vascular health. Many methods rely on invasive procedures, such as arteriograms, to determine whether vascular stenosis is occurring. These invasive techniques are often not ordered until the patient becomes symptomatic. For example, carotid arteriograms can be ordered following a physical examination pursuant to the appearance of a clinical symptom. Performing an arteriogram is not without risks due to the introduction of dye materials into the vascular system that can cause allergic responses. Arteriograms also use catheters that can damage the vascular wall and dislodge intraluminal plaque, which can cause an embolic stroke at a downstream site. It would therefore be useful to develop a noninvasive or limited invasive procedure for assessing vascular health.
Further, in the field of hemodialysis and other techniques where blood is removed from a patient for processing and then returned, it is important to periodically assess the blood flow rate through an arteriovenous fistula, graft, or catheter to monitor the onset of stenosis. This is often accomplished by the reading of access pressures through the venous and arterial access needles. Early detection of stenosis associated with the placement of a fistula, graft, implantable port, or a catheter can permit low cost repairs to be made. On the other hand, if these problems are ignored or not detected, the cost of the revision or replacement of the fistula, graft, implantable port, or catheter can be very high and burdensome to the patient.
There have been several devices that have been developed to determine pressure inside a dialysis machine or during hemodialysis. For example, as disclosed in U.S. Pat. No. 5,454,374 to Omachi, access pressures can be determined through volumetric manipulations involving the determination of a pressure head height of blood in a visual manner. The blood line going to the dialysis machine is used to measure pressure and the problem is one of determining the height between the transducer and the patient's access site.
U.S. Pat. No. 4,710,163 to Levin et al. discloses a method and system for continuously monitoring patient heart rate and mean arterial blood pressure during hemodialysis and for automatically controlling fluid extraction rate and/or dialysate sodium concentration in the event that blood pressure and/or heart rate indicate onset or impending onset of a patient hypotensive episode. There are three separate machines for performing these functions: an automated blood pressure monitor, an automated patient heart rate monitor, and the hemodialysis machine. The blood pressure monitor is essentially a device for measuring blood pressure based on the blood in the patient's arm, i.e. a cuff that inflates and deflates automatically to read the diastolic and systolic blood pressure readings. This device merely takes the place of an actual technician to take a blood pressure reading. The blood pressure readings are derived from a standard blood pressure cuff on the patient's arm and not from the intravascular blood near the access site for an extracorporeal circuit.
U.S. Pat. No. 6,623,443 to Polaschegg discloses a device that measures and compares the amplitude of pressure pulses within an extracorporeal circuit to determine whether a stenosis has occurred therein. The peak-to-peak amplitude of the pressure waves created by variations in the patient's blood pressure and variations in pressure created by the extracorporeal blood pump are used to indicate the presence of an obstruction in the circuit. A deviation in the peak-to-peak amplitude of the pressure signal from a predetermined standard value indicates a stenosis or loss of occlusion of the roller pump. No standard is defined to indicate a stenosis that represents a significant risk to the patient. No measurements or calculations of intravascular blood pressure occur.